This invention relates to the production of polyamides, particularly the production of poly(epsilon-caprolactam), known as nylon 6, shaped articles such as filaments produced directly from freshly prepared (nascent) polymer. More particularly, the invention relates to an improvement in the first or hydrolysis step of polymerization, namely, limiting the amount of free water fed to the polymerization process and limiting catalyst being fed to the polymerization process to a specific range.
The apparatus preferred for the practice of the process of this invention is that shown in FIG. 3 of U.S. Pat. No. 4,310,659 or alternatively, using the hydrolyzer shown in the figure of U.S. Pat. No. 3,813,366 to replace the two-stage hydrolyzer labeled 1 and 2 in FIG. 3 of U.S. Pat. No. 4,310,659. The above patents, particularly the drawings of each, are hereby incorporated by reference.
Previous nascent, or polymerization directly coupled to spinning, processes such as that described in U.S. Pat. No. 3,813,366 and in U.S. Pat. No. 3,578,640, also hereby incorporated by reference, produced shaped articles such as filaments (fiber) having higher water extractables content than the similar articles (fiber) produced from polymer chips which were water washed and dried before being melted and shaped as by spinning. Such a batch process is disclosed in U.S. Pat. No. 3,242,134, hereby incorporated by reference, which also discloses the use of manganous chloride and hypophosphorus acid dissolved in caprolactam to improve light stability of the resulting nylon filaments. A particularly troublesome water extractable component to processes of shaped articles (fiber) are cyclic oligomers of caprolactam. Present directly coupled (nascent) processes produced fiber or other shaped articles having a cyclic dimer content of about 0.15 to 0.3 percent compared to fiber or shaped articles produced from washed, dried and melted chips of about 0.1 percent cyclic dimer content. See the discussion at column 1 of U.S. Pat. No. 4,310,659 and also U.S. Pat. No. 3,287,322, hereby incorporated by reference, for discussion of oligomers and their effects in yarn.
The definition of "water extractables", "oligomer", and "polymer" is found in U.S. Pat. No. 4,310,659, which is hereby incorporated by reference. Total oligomer content is difficult to measure. However, cyclic dimer content can be routinely measured. Thus, the cyclic dimer content discussed herein provides a rough indication of the total oligomer content of the polymer. It is commercially known to practice the invention described in U.S. Pat. No. 4,310,659 or U.S. Pat. No. 4,813,366, details of the final finisher used are disclosed in U.S. Pat. Re. No. 28,937, hereby incorporated by reference, using manganous hypophosphite as an additive to the hydrolyzer with a water content as low as 1.5 percent. It is also commercially known to practice the method disclosed in U.S. Pat. No. 3,242,134 with lower water content, see, for example, Example 2. Also, U.S. Pat. No. 3,425,986 discloses the use of metal pyrophosphate with water content as low as 0.1 to 1.5 percent. The above listed prior art is hereby incorporated by reference as is the following less pertinent prior art: U.S. Pat. No. 3,779,998 discloses hypophosphorus acid as a catalyst but discloses a reduced viscosity of the resulting polymer. U.S. Pat. No. 3,579,483 discloses use of hypophosphorus acid and manganese chloride but does not teach the water content of the feed polymer. U.S. Pat. No. 2,564,001 discloses a much higher percent of hypophosphorus acid as catalyst. U.S. Pat. No. 3,173,898 discloses the manganese salt and U.S. Pat. No. 2,640,044 and U.S. Pat. No. 2,705,227 teach the use of the hypophosphorus acid catalyst but require other additives or make no teaching of the water content. Also of interest are U.S. Pat. No. 3,352,821; U.S. Pat. No. 4,111,869, which requires an anhydrous system; U.S. Pat. Nos. 3,211,705; 3,328,342; 3,330,802; 2,887,462; 2,564,001; 3,594,346; 3,640,948; 4,097,469; 3,002,947; 3,893,973; 3,839,380; 2,557,808; G.B. Pat. Nos. 1,193,866 and 663,761.
The following literature teaches use of low free water content polymerization of nylon at different or no catalytic phosphorus levels.
Water content between 0.76 and 2.12% with no other catalyst is taught in sealed tube studies by Kazuo Tai et al. in J. of Appl. Polymer, Science, Vol. 24, pages 211-224 (1979), John Wiley.
D. Heikens et al. teach water content between 0.8 and 1% with no catalyst in J. Polymer Sci., Vol. XLIV, pages 437-448 (1960).
In Khimicheskie Volokna, No. 1, pages 29-31 (1977), Khaitin et al. teach 0.9 to 3.6% water with no catalyst.
Furukawa et al. in Kobanshi Ronbanshu, Vol. 37, No. 8, pages 535-541 teach a series of phosphorus compounds, including phosphoric, at 310 ppm with 0.2% water in sealed tube studies.
Mizerovski et al. teach in Khimicheskie Volokna, No. 3, page 69 (1973) 1400 ppm of H.sub.3 PO.sub.4 with 0.5% water in sealed tube studies.